This invention relates to the use of bis(perfluoroalkanesulfonyl)imide and its salts as surfactants (surface active agents) or additives in aqueous and nonaqueous (or organic) solutions. More particularly, this invention relates to bis(perfluoroalkanesulfonyl)imides and their salts as surfactants or additives in solutions for applications having an extreme environment.
Surfactants or surface active agents are generally described as any compound that reduces surface tension when dissolved in a solution (e.g., water) or that reduces interfacial tension between two liquids or between a liquid and a solid. An additive is defined herein as any compound that when added to a base hydraulic fluid improves the erosion performance of that hydraulic fluid. The choice of surfactant/additive varies with the application, system, or solution where it is employed. Some factors that are considered when evaluating the utility of a surfactant/additive include: solubility of the surfactant/additive in the system used, pH, ultimate surface tension required for spreading and/or for leveling, how corrosive the system components are, conductivity required, and volatility of the surfactant.
Surfactants are used in a variety of applications including aqueous and non-aqueous (i.e., organic) solutions. Applications that involve an extreme or a harsh environment include, but are not limited to, etching solutions used in preparation of semiconductor devices, electrochemical plating and polishing solutions, wafer cleaning and polishing solutions, anisotropic etching solutions, electrolytes for alkaline batteries, and developer solutions for semiconductor manufacture. An xe2x80x9cextremexe2x80x9d or xe2x80x9charshxe2x80x9d environment is defined as an application where a conventional hydrocarbon-based surfactant/additive would be degraded. Degradation is defined herein as protonation of an anionic surfactant or degradation or alteration (e.g., polymerization) of the chemical structure of the surfactant that results in a change in surface tension and thus diminished surfactant performance.
Extreme environment conditions for aqueous solutions include, but are not restricted to, heat, light, radiation, oxidizing/reducing potential, high pressure, high shear, high or low pH, or the presence of corrosive reagents. The pH is generally low (i.e., acidic) or high (i.e., basic). Typically, for aqueous solutions, these applications are minimally both extreme in pH and corrosive.
Extreme environment conditions for non-aqueous solutions include, but are not restricted to, high pressure, high temperature (i.e., at least 200xc2x0 C.), high shear, environmental (trace water), or corrosive reagents, etc.
Under these conditions, most hydrocarbon surfactants either chemically or physically degrade. Thus, non-hydrocarbon-based surfactants that do not degrade in extreme environments are desirable.
Fluorocarbon surfactants such as, perfluorooctanesulfonyl fluoride (POSF) derived surfactants like K+xe2x88x92OSO2C8F17 have been used. But these surfactants persist or ultimately degrade in the environment to other perfluorooctyl-containing compounds. It has been reported that certain perfluorooctyl-containing compounds may tend to have low bioelimination rates in living organisms. This tendency has been cited as a potential concern for some fluorochemical compounds. See, for example, U.S. Pat. No. 5,688,884 (Baker et al.). Introduction of a surfactant and its degradation products into the environment may occur either from streams related to the use of the surfactant itself or to waste streams/emissions from its manufacture.
As a result, there is a desire for fluorine-containing compositions that are effective in providing desired surfactant properties, and that eliminate more rapidly from the body (including the composition and its degradation products).
Therefore, the need exists for a surfactant or an additive that works effectively in an extreme environment application, that more rapidly eliminates from living organisms, and that can be manufactured in a cost effective manner. The precursor to these surfactants needs to be less bioaccumulative than POSF derived materials to prevent potentially introducing bioaccumulative materials from manufacturing streams.
The present invention provides bis(perfluoroalkanesulfonyl)imides and their salts as surfactants or additives in applications having extreme or harsh environments. Advantageously, the surfactants or additives of the present invention are comprised of a perfluorinated imide anion, xe2x88x92N(SO2CnF2n+1)2, that is physically and chemically stable while providing outstanding surface activity under chemically harsh or extreme conditions. The surfactants or additives of the present invention furthermore do not readily protonate (even in aqueous solutions of low pH), commonly encountered in many extreme applications due to the very low basicity of the perfluorinated anion. In addition, the surfactants of the present invention can be produced from lower cost intermediates, such as CF3SO2F and C4F9SO2F, compared to surfactants derived from C8F17SO2F (POSF), which is produced in relatively low yields by ECF. Surprisingly, the replacement of perfluorooctanyl based surfactants with surfactants of the present invention is possible without sacrificing surface activity or surfactant performance. Moreover, the surfactants of the present invention will likely more rapidly bioeliminate.
In one aspect, the present invention comprises a method of reducing the surface tension of a solution comprising: (a) providing an aqueous solution that would degrade a hydrocarbon-based surfactant; and (b) adding to said aqueous solution a surface active agent comprising a bis(perfluoroalkanesulfonyl)imide or one of its salts.
Another embodiment of the present invention is a hydraulic fluid comprising: one or more phosphate esters; at least one bis(perfluoroalkanesulfonyl)imide salt additive; and optionally comprising adjuvants selected from the group consisting of viscosity index improvers, acid control components, antioxidant components, lubricants, dyes, corrosion inhibitors, antifoamers and mixtures thereof.
Another aspect of the present invention is an application having an extreme environment wherein said application comprises an aqueous solution comprising a bis(perfluorobutanesulfonyl)imide or one of its salts.